Popular kinds of compressor or vacuum pump have sliding vanes or screws that are lubricated with oil. The oil is used to seal the working faces of the compressor or pump to allow a more efficient compression cycle. Because of the action of the machine, it is inevitable that a finely divided mist of oil droplets will enter the air stream after the compression cycle. Consequently oil appears downstream of the compressor or vacuum pump in the form of an aerosol whose particle size has been found to range between 0.01 and 5 microns. Primary oil separation is normally effected by impingement of the air stream onto a surface, for example using a series of baffles or using a spinner mechanism as shown in our British Patent Application No. 8703314 and up to 95% of the entrained oil can be removed in this way. The air containing the residual oil aerosol passes to a coalescing filter for a second stage of oil removal.
Coalescing filters for secondary oil separation are usually tubular and generally have a coalescing layer which fits within or has within it (depending on the direction of flow) a drainage sleeve, the oil draining from the sleeve which is necessarily of a much coarser nature. Such a filter will desirably combine a minimum resistance to air through flow with a minimum oil carry-over.